1. Field of the Invention
This invention relates to an acceleration slip control device for a vehicle by which a slipping of the driving wheels of the vehicle during a start or acceleration of the vehicle is controlled.
2. Description of the Related Art
An acceleration slip control device is commonly used to improve the acceleration and straight line running stability of the vehicle by preventing an excess slippage of the driving wheels during a start or acceleration of the vehicle.
Usually, the acceleration slip control device determines a target speed for the speed of rotation of the driving wheels and controls a drive torque of the driving wheels by a feedback control in such a manner that the deviation of the speed of rotation of the driving wheels from the target speed is maintained at less than a predetermined value.
In such a control, to improve a response and stability of the control, the drive torque is controlled in such a manner that the rate of change of the drive torque is changed according to the magnitude of a friction factor .mu. between the driving wheels and a road surface.
For example, when .mu. is low (i.e., on a slippery road), the rate of increase of the drive torque is made smaller, to prevent an overshoot and thereby increase the stability of the control. Conversely, when .mu. is high, the rate of increase of the drive torque is made larger, to improve the response of the control.
An example of an acceleration slip control device of this type is disclosed in Japanese Unexamined Patent Publication No. 60-99757.
The device disclosed in this Publication calculates the acceleration of the vehicle from the rate of change of the speed of the driven wheels (i.e., a running speed of the vehicle) when a slip occurs between the driving wheels and a road surface. Then the device calculates the friction coefficient between the driving wheels and the road surface from this acceleration, and adjusts the drive torque in accordance with the calculated friction coefficient by a fuel cut operation, etc.
In the acceleration slip control device of the above Publication however, the detected values of the friction coefficients are sometimes different from the actual friction coefficients, especially when frequent changes occur in the road surface conditions. Also errors in detecting the speed of driven wheel can occur, and these errors adversely affect the calculation of the acceleration, and consequently, the calculation of the friction coefficient.
When the acceleration control is carried out on the basis of a friction coefficient higher than the actual friction coefficient, the rate of increase of the drive torque becomes extremely large, and therefore, the stability of the control is worsened because of an increased slip between the driving wheels and the road surface. Conversely, when the control is carried out on the basis of a friction factor lower than the actual friction coefficient, the rate of increase of the drive torque becomes very small, and thus the response of the control, and the acceleration, are worsened.